Simultaneous Recognition and Separation of Organic Isomers Via Cooperative Control of Pore‐Inside and Pore‐Outside Interactions

Abstract Despite the desirability of organic isomer recognition and separation, current strategies are expensive and complicated. Here, a simple strategy for simultaneously recognizing and separating organic isomers using pillararene‐based charge‐transfer cocrystals through the cooperative control of pore‐inside and pore‐outside intermolecular interactions is presented. This strategy is illustrated using 1‐bromobutane (1‐BBU), which is often produced as an isomeric mixture with 2‐bromobutane (2‐BBU). According to its structure, perethylated pillar[5]arene (EtP5) and 3,5‐dinitrobenzonitrile (DNB) are strategically chosen as a donor and an acceptor. As a result, their cocrystal exhibited stronger pore‐inside interactions and much weaker pore‐outside interactions with 1‐BBU than with 2‐BBU. Consequently, nearly 100% 1‐BBU selectivity is achieved in two‐component mixtures, even in those containing trace 1‐BBU (1%), whereas free EtP5 only achieved 89.80% selectivity. The preference for linear bromoalkanes is retained in 1‐bromopentane/3‐bromopentane and 1‐bromohexane/2‐bromohexane mixtures, demonstrating the generality of this strategy. Selective adsorption of linear bromoalkanes induced a naked‐eye‐detectable color change from red to white. Moreover, the cocrystal are used over multiple cycles without losing selectivity.


Vaporhromic experiments
An open 4 mL vial containing 20 mg of E-D cocrystal or EtP5 was placed in a sealed 20 mL vial containing 1 mL of each bromoalkane solution (2). E-D or EtP5 powders were exposed under saturated vapor pressure in the closed vessel at room temperature. Obvious color changes of E-D powders were observed over time. Obvious color changes of EtP5 powders were not observed over time. 1 H NMR, 13 C NMR, and 2D NOESY spectroscopy were measured in CDCl 3 using a Bruker Avance III 400 MHz nuclear magnetic resonance spectrometer. 13 C CPMAS NMR was performed at a static magnetic field of 9.4 T with a Bruker Avance III wide-bore NMR spectrometer with a Bruker 4 mm probe at a 1 H NMR frequency of 600 MHz.

Solid-state UV-Vis spectra
Solid-state UV-Vis spectra were recorded on a SHIMADZU UV-2600 spectrophotometer.

Thermogravimetric Analysis
DSC-TG was carried out using a Mettler Toledo-TGA/DSC3+ instrument and the samples were heated under air gas at a rate of 10 °C / min.

Powder X-ray diffraction (PXRD)
Powder X-ray diffraction patterns were obtained at ambient (30 °C) temperature by a Bruker D2 with Cu-Kα radiation (λ = 1.5406Å) at 30 kV and 10 mA. The samples were placed onto a circular sample holder and leveled with a glass slide. The sample was scanned within the scan range of 2θ from 5° to 50° continuous scan with a step size of 0.02° and a scan speed of 0.2 s per step.

Fourier Transform Infrared Spectroscopy (FT-IR)
The FT-IR spectra have been measured on a Perkin Elmer 480 FT-IR spectrophotometer (KBr pellet).

Raman Spectroscopy
The Raman spectroscopy has been measured on a Horiba scientific-LabRAM HR evolution.

Optical images
Optical images were taken with a Canon 70D camera.

Electrochemical Characterization
The Mott-Schottky spots were carried out with CHI 660E electrochemical workstation. A 10 mg powder sample was weighed and dispersed in 1 mL of ultrapure water, then an appropriate amount of conductive binder was added and sonicated for 30 min to form a homogeneous suspension, then 150 L of the suspension was added dropwise on the FTO glass and dried at room temperature. The test was performed with a three-electrode system, with FTO as the working electrode, silver/silver chloride as the reference electrode, and platinum sheet as the counter electrode. 0.2 M Na 2 SO 4 aqueous sodium was used as the electrolyte, and the photoelectric test was performed under a nitrogen atmosphere. The conductive binder is selected from polyvinyl alcohol or sodium carboxymethyl cellulose, and the test frequency is selected from 500, 1000, and 1500 Hz.

EtP5 selective adsorption of BBU
An open 4 mL vial containing 20 mg of EtP5 was placed in a sealed 20 mL vial containing 1 mL of each bromoalkane solution. EtP5 powders were exposed under saturated vapor pressure in the closed vessel at room temperature. Obvious color changes were not observed over time. All bromoalkanes isomer mixtures are volume ratio, such as 1:1 (v/v), 1:99 (v/v), 99:1 (v/v). Before measurement, the powders were heated at 40 °C to remove the surface-physically adsorbed vapor.         Figure S10. DSC-TG of EtP5 (air atmosphere). EtP5 loses weight in the air in two stages, the first stage is the consumption of its O atoms, and the second stage is the consumption of air, corresponding to 38 %, and 62 %, respectively. The melting point of EtP5 is 160.67 °C.

Choosing a matching acceptor
We calculated the intermolecular binding energy and electrostatic potential, using the DFT-D3 method (DFT including the D3 version of Grimme's dispersion) at the B3LYP/6-31G level to predict their pore-outside intermolecular interactions.   Figure S16. The FT-IR of E-D-a cocrystal, EtP5, and DNB.

Characterization of E-D-a Cocrystal
After the formation of cocrystal between EtP5 and DNB, the CN group of DNB was shifted from 2247 cm -1 to 2241 cm -1 , suggesting charge transfer interaction between EtP5 and DNB.

Selective separation of bromoalkane isomers 9.1 Time-dependent solid-vapor adsorption experiment of v(1-BBU:2-BBU) = 1:1 mixed vapor for E-D cocrystal
An open 4 mL vial containing 20 mg of E-D powders was placed in a sealed 20 mL vial containing 1 mL of each bromoalkane solution. E-D powders were exposed under saturated vapor pressure in the closed vessel at room temperature. All bromoalkane isomers mixtures are volume ratio 1-BBU:2-BBU 1:1 (v/v). Before measurement, the powders were heated at 40 °C to remove the surface-physically adsorbed vapor.

Characterization of E-D cocrystal after adsorption 1-BBU:2-BBU mixed vapor
An open 4 mL vial containing 20 mg of E-D cocrystal was placed in a sealed 20 mL vial containing 1 mL of each bromoalkane solution. E-D powders were exposed under saturated vapor pressure in the closed vessel at room temperature. Obvious color changes were observed over time. All bromoalkanes isomer mixtures are volume ratio, such as 1:1 (v/v), 1:99 (v/v), 99:1 (v/v). Before measurement, the powders were heated at 40 °C to remove the surface-physically adsorbed vapor.